1. Field of the Invention
The present invention relates to an outboard motor.
2. Description of the Related Art
US2013/0052891A1 discloses an outboard motor including a gear case disposed underwater and a gear mechanism housed inside the gear case and lubricated by a lubricating oil. The lubricating oil is stored in a gear chamber provided in the gear case. The lubricating oil inside the gear chamber is thrown upward by rotation of a bevel gear included in the gear mechanism and fed upward to an oil slinger provided at a driveshaft. The lubricating oil suppled to the oil slinger is guided upward along a spiral oil groove by rotation of the driveshaft. The lubricating oil is thus supplied to an internal gap of a tapered roller bearing that rotatably supports the driveshaft. The lubricating oil that has passed upwardly through the tapered roller bearing is returned to the gear chamber by a main lubrication circulation portion and a sub lubrication circulation portion.
JP S57182595 A discloses an example of a conventional circulation path for a lubricating oil. As in US2013/0052891A1, the lubricating oil inside a gear chamber of JP S57182595 A is fed upward by rotations of a pinion, a forward drive gear, and a reverse drive gear. The lubricating oil is fed from the gear chamber to an insertion space via a lower communication passage extending upward from the gear chamber. A sleeve surrounding a driveshaft is disposed at the insertion space. The lubricating oil supplied to the insertion space enters inside the sleeve from an opening portion that opens at an outer peripheral surface of the sleeve and is fed upward by a spiral lead portion provided on an inner peripheral surface of the sleeve. The lubricating oil is supplied to an upper bearing via an oil feed portion extending upward from the lead portion. The lubricating oil supplied to the upper bearing flows downward between an inner peripheral surface of the insertion space and the outer peripheral surface of the sleeve and is discharged from the insertion space via an upper communication passage positioned at a height between the upper bearing and the lead portion. An outer diameter of a passage defined between the inner peripheral surface of the insertion space and the outer peripheral surface of the sleeve is greater than an outer diameter of the driveshaft.
With a conventional outboard motor, a lubricating oil is circulated in a circulation path provided in an interior of a lower case that is disposed underwater. Gears, bearings, etc., are thus lubricated and these members are cooled by the lubricating oil. The lubricating oil that has cooled the gears, etc., is cooled by the lower case, etc., while circulating through the circulation path. A temperature of the lubricating oil is thus maintained within an appropriate range. However, since a heat amount generated at the lower case increases as gears and other rotating bodies are made larger and outboard motors are made higher in output, a temperature increase of the lubricating oil must be minimized to prevent the temperature of the lubricating oil from exceeding the appropriate range.
A temperature increase of the lubricating oil is suppressed by increasing a total amount of the lubricating oil circulating inside the lower case because a heat capacity of the lubricating oil as a whole is thus increased. Also, a temperature increase of the lubricating oil is decreased without increasing the total amount of the lubricating oil by improving circulation efficiency of the lubricating oil, that is, by preventing stagnation of the lubricating oil or increasing a circulation flow rate of the lubricating oil because heat radiation from the lubricating oil will then be performed effectively.
However, with the method of increasing the total amount of the lubricating oil, an oil storage chamber storing the lubricating oil inside the lower case must be enlarged. Depending on the outboard motor, it may not be possible to provide a lower case with such a large oil storage chamber. Also, when the lower case is enlarged to enlarge the oil storage chamber, the outboard motor is lowered in propulsion efficiency because of increased resistance of water applied to the outboard motor.
With the outboard motor of US2013/0052891A1, two lubrication circulation portions (the main lubrication circulation portion and the sub lubrication circulation portion) that return the lubricating oil that has lubricated the tapered roller bearing to the gear chamber are provided to smoothly circulate the lubricating oil without enlarging the gear case. However, the circulation flow rate of the lubricating oil is dependent on a supply capacity of the oil slinger that feeds the lubricating oil to the tapered roller bearing and therefore the lubricating oil cannot be circulated at a flow rate exceeding the supply capacity of the oil slinger. That is, unless the lubricating oil supply capacity at the oil slinger is increased, the circulation flow rate of the circulation system as a whole cannot be increased even if the sub lubrication circulation portion is added.
A flow rate of the lubricating oil supplied to the oil slinger by rotation of the bevel gear and a flow rate of the lubricating oil fed by the oil slinger both increase with increase of engine speed. However, during high speed rotation, the flow rate of the lubricating oil supplied to the oil slinger by rotation of the bevel gear becomes greater than the flow rate of the lubricating oil fed by the oil slinger. Even when the flow rate of the lubricating oil supplied to the oil slinger exceeds the flow rate of the lubricating oil fed by the oil slinger, the lubricating oil will not be circulated at a flow rate exceeding the supply capacity of the oil slinger. Further, in this case, the lubricating oil stagnates between the bevel gear and the oil slinger, so that a pressure of the lubricating oil increases and the temperature of the lubricating oil increases.
Similarly with the outboard motor of JP S57182595 A, the lubricating oil cannot be circulated at a flow rate exceeding a supply capacity of the spiral lead portion.